1. Field of the Invention
The present invention relates to a printing apparatus for forming an image by discharging ink to a printing medium on the basis of input image information, a control method therefor, and a computer-readable memory.
2. Description of the Related Art
In resent years, OA devices such as personal computers, copying machines, and wordprocessors have become popular. As a kind of printing eapparatuse for these devices, inkjet printing apparatuses for printing an image by an inkjet printing method are rapidly developed and spread. With advanced functions of OA devices, color images are required, and various color inkjet printing apparatuses are being developed.
In general, the inkjet printing apparatus comprises a printing means (printhead), a carriage for mounting an ink tank, a convey means for conveying a printing sheet, and a control means for controlling them. A printhead for discharging ink droplets from a plurality of orifices serial-scans in a direction (to be referred to as a main scanning direction) perpendicular to the convey direction (to be referred to as a subscanning direction) of a printing sheet. In non-printing, a printing sheet is intermittently conveyed by an amount equal to a printing width. A color inkjet printing apparatus forms a color image by overlapping ink droplets discharged from printheads of a plurality of colors on a printing medium.
Examples of the method of printing an image by discharging ink in the inkjet printing apparatus area method using an electrothermal energy converter in which a heating element (electrothermal energy converter) is disposed near an orifice and an electrical signal is applied to the heating element to locally heat ink and change the pressure, thereby discharging ink from the orifice, and a method using an electro-mechanical converter such as a piezoelectric element. A known example of the means of discharging ink is an arrangement using an electro-pressure conversion means, such as a piezoelectric element, to apply a mechanical pressure to ink, thereby discharging the ink.
These methods print characters and figures by discharging small ink droplets from an orifice onto a printing medium in accordance with print data. The inkjet printing apparatus hardly generates noise because of a non-impact type, can reduce the running cost and apparatus size, and can relatively easily print a color image. With these advantages, the inkjet printing apparatus is employed in a computer, wordprocessor, and the like. Further, the inkjet printing apparatus is widely used as a printing apparatus mounted on a stand-alone copying machine, printer, facsimile, and the like.
In the printing method of the conventional inkjet printing apparatus, a dedicated coated sheet having an ink absorption layer must be used to obtain a high-development color image free from any ink blur on a printing medium. Recent improvements of ink and the like allow practically using a method having printability on plain sheets which are enormously consumed in a printer, copying machine, and the like. In addition, demands arise to cope with various printing media having different ink absorption characteristics, such as an OHP sheet, cloth, and plastic sheet. To meet these demands, printing apparatuses capable of performing best printing regardless of the type of printing medium are being developed and put into practical use. As for the size of a printing medium, demands arise for printing on a large-size printing medium such as printing on an advertising poster, cloth such as clothes, and the like. Such inkjet printing apparatus is being demanded as an excellent printing means in various industrial fields. Higher image qualities and higher speeds are also being required.
In general, the printing method of the color inkjet printing apparatus realizes color printing using three, cyan (Cy), magenta (Mg), and yellow (Ye) color inks or four color inks including a black (Bk) ink. This color inkjet printing apparatus prints a color image, unlike a monochrome inkjet printing apparatus mainly used to print characters, and is required for various factors such as the color development, gradation, and uniformity of an image to be printed.
However, the quality of an image to be printed greatly depends on the performance of the printhead itself. That is, slight differences between orifices caused in manufacturing the printhead, such as variations in shapes of the orifices of the printhead or electrothermal converters (discharge heaters), influence the discharge amount and direction of discharged ink, resulting in low image quality as density nonuniformity of a final printed image. Consequently, a "blank" portion which inhibits an area factor of 100% periodically appears in the main scanning direction, dots excessively overlap each other, or a blank stripe appears on a printing medium. These phenomena are sensed as density nonuniformity by a human eye.
To prevent this density nonuniformity, a multipath printing methodisproposed. This multipathprinting method will be described with reference to FIG. 17.
In FIG. 17, a multipath printing method using a printhead of a single ink color having eight nozzles (orifices) will be exemplified for descriptive convenience.
FIG. 17 is a view for explaining the conventional multipath printing method.
In the first scanning of the printhead in the main scanning direction, a staggered pattern .circle-solid. is printed using first four nozzles out of the eight nozzles of the printhead. The printing sheet is fed in the subscanning direction by half the printing width of the printhead (by a width of 4 dots in this case). Then, in the second scanning of the printhead, an inverted staggered pattern .largecircle. is printed using all the eight nozzles of the printhead to complete printing in a printing area corresponding to half the printing width of the printhead. That is, a 4-dot wide printing area is completed every scanning by sequentially feeding the printing sheet in units of 4 dots and alternately printing staggered and inverted staggered patterns. In this way, one line (printing area by one scanning with the printing width of the printhead) is printed using two different nozzles, thereby forming a high-quality image almost free from density nonuniformity. Also, the multipath printing method can perform printing while drying ink.
Known examples of a method of generating data (path data) not to be printed (not to discharge ink) in each scanning are a method (fixed thinning method) of generating path data by thinning out print data using a staggered/inverted staggered pattern, as described above, a method (random thinning method) of generating path data by thinning out print data using a random mask pattern prepared by laying out print dots and non-print dots at random, and a method (data thinning method) of generating path data by thinning out print dots.
To form an image of a color other than three, cyan (Cy), magenta (Mg), and yellow (Ye) color inks or four color inks including a black (Bk) ink on a printing medium, ink droplets of a plurality of colors are landed on the same position to mix the colors on the printing medium. An example of printing green (G) with a printhead constituted to perform printing on a printing medium in the order of Bk, Cy, Mg, and Ye in forward-path scanning and in the order of Ye, Mg, Cy, and Bk in return-path scanning will be explained with reference to FIG. 18.
FIG. 18 is a view for explaining a conventional dot landing surface of two colors.
As shown in FIG. 18, when another dot overlaps a previously printed dot in inkjet printing, the subsequent dot tends to sink in a sheet surface deeper than the previous dot at the overlapping portion.
To print green (G), Cy and Ye inks are sequentially landed on a printing medium in forward-path scanning by the carriage. At this time, the Cy ink permeates into the printing medium and spreads on the surface and internally. The Ye ink landed next gets under the Cy ink. When viewed from the printing medium surface, the Ye ink seems to spread around the Cy ink. The mixed portion of Cy and Ye becomes G, and is recognized by a human eye as if G were printed. To the contrary, in return-path scanning, the Ye and Cy inks are sequentially landed. The Cy ink gets under the Ye ink, and the mixed portion of Ye and Cy becomes G' which contains Ye more dominantly than Ye. That is, the preferential color changes depending on the landing order of two types of inks (Cy and Ye in this case). On the forward path, previously absorbed Cy is a preferential color to obtain Cy-rich G. On the return path, Ye-rich G' is obtained.
For this reason, in reciprocal printing, even color mixing of the same Cy and Ye inks changes the tint between mixed colors by forward-path printing and return-path printing. As a result, two different colors are expressed on a printing medium with respect to human visual properties, and mixed colors having different tints are recognized every scanning.
To solve this problem, there is proposed a color inkjet printing apparatus having two independent printheads for forward-path printing and return-path printing, thereby realizing high-quality reciprocal printing free from any tint difference. In this color inkjet printing apparatus, a printhead for forward-path printing and a printhead for return-path printing have symmetrical (opposite) printhead layouts of cyan (Cy), magenta (Mg), yellow (Ye), and black (Bk) colors, and thus have a common landing order of the respective inks to a printing medium to eliminate the tint difference between mixed colors by forward-path printing and return-path printing.
However, in the printing apparatus constituted to eliminate the tint difference between mixed colors by forward-path printing and return-path printing, the forward-path printhead performs printing operation on only the forward path and lies idle on the return path. Similarly, the return-path printhead lies idle on the forward path and performs printing operation on only the return path. That is, although the printing apparatus comprises the two printheads, one printhead performs only one-way printing, which poses the following problem.
While the printhead lies idle in non-printing scanning, it must form an image at a high duty in printing scanning. This prevents an increase in carriage speed or causes a discharge error owing to the temperature rise of the printhead. In terms of forming a high-quality image, the two printheads are not effectively used.